In power-line communications (PLC), utility power lines, especially the high-voltage (HV, 60 kVAC and up) and medium-voltage (MV, 4–35 kVAC) power lines, are used as a transmission medium. The MV power lines are generally used to power the primaries of distribution transformers feeding electric power to homes and businesses. It is advantageous to convey communication signals in radio frequencies.
A typical scenario in PLC is shown in FIG. 1. As shown, a main power line L1 and a number of other power lines L2, L3, L4 branching off from L1 are used to carry the RF communication signals. A server 10 is used at a distribution center to receive multimedia information from service providers and to send the information to a plurality of customers downstream. The server 10 uses an RF coupler 12 and an associated distribution modem 11 to broadcast the RF communication signals on power line L1 so that the customers can receive the signals using their customer premise equipment (CPE). For example, CPE 20 and CPE 30 acquire the RF signals from L1 via RF couplers 22, 32 and associated modems 21, 31 while CPE 40 acquires the RF signals from L3 via a RF coupler 42 and an associated modem 41, and so on. In the upstream direction, customers can use their CPE to send request data to the server via the same couplers and modems.
RF signals are attenuated considerably as they are transmitted along the power line. As a result, a CPE located too far from the server 10 may not be able to receive usable RF signals. For example, while CPE 20 may be able to receive good signals from the server 10, CPEs 30, 40 and 50 may not. Thus, it is necessary to provide a plurality of repeaters 72, 74, . . . along the power lines to make it possible for CPE 30, 40 and 50 to receive the communication signals.
RF signals transmitted along the power line are susceptible to a variety of interference sources. For example, interfering signals from electrical impulses, amateur radio bands and AM radio broadcasting can corrupt the RF signals, rendering the detection and demodulation of RF signals ineffective. It is thus advantageous and desirable to provide a method and device to reduce the effects of signal interference.